1. Field of the Invention
The present invention relates to new sheet molding compounds (hereafter referred to as SMCs) with one side having the properties of a low-specific-gravity SMC and the other side having the properties of a high-strength SMC so as to serve for production of lightweight molded material, and further relates to processes for producing them, and also to lightweight, high-strength molded material of a sandwich structure which is produced by press-molding a combination of such SMCs.
2. Description of the Prior Art
As widely known, fiber-reinforced thermosetting plastics (hereafter referred to as FRP) have many advantages, including light weight, high strength and high corrosion resistance, over metals (iron in particular), and have been used for a variety of applications including bath tubs, sewage tanks, piped,boats, chairs, and various automobile components. There are many different processes for molding of FRP. One of the important processes is press molding, in which material for FRP is put in a press tool maintained at a required temperature, and then pressed and cured. Either liquid resin or molding material such as SMC is generally used as material for FRP. In particular, pressing of molding material is widely practiced to produce molded FRP because of various advantages including a smaller material loss, higher productivity and superior working environment as compared with other FRP molding processes.
Though having these good features, SMCs are lower in specific rigidity than light metals including aluminum and therefore, application of an SMC to interior and exterior automotive trims, including sunroof housing, designed to have a high rigidity would not be beneficial with respect to reduction in weight. To solve this problem, there have been efforts seeking to develop low-specific-gravity SMCs containing hollow, lightweight filler particles. However, though an increased content of hollow lightweight filler particles can decrease the specific weight, it may cause large deterioration in mechanical properties (flexural modulus in particular). Thus, SMCs comparable with aluminum in terms of specific rigidity have not been developed as yet. The above-mentioned low-specific-gravity SMCs can be improved in strength by using such SMC reinforcing material as carbon fiber or aramid fiber which are higher In tensile strength than glass fiber commonly used. It is difficult, however, for this process to produce practical SMCs at low cost. The present inventors have already invented and filed a patent application for molded material with increased specific rigidity which consists of a low-specific-gravity SMC layer sandwiched between two high-strength SMC layers, with the high-strength SMC layers serving to reinforce the low-specific-gravity SMC layer which is inferior in mechanical properties.
Later, the inventors made further studies seeking to improve the properties of the above-mentioned molded material, and identified some problems to be solved, including insufficient adhesion at the interfaces between the low-specific-gravity SMC layer and high-strength SMC layers, and increased thickness due to the multilayer structure.